1. Field of the Invention
The present invention relates to a chain tensioner including a tensioner body having a cylindrical plunger housing hole that is open at one end, a cylindrical plunger slidably inserted into the plunger housing hole, and bias means housed, in an extensible manner, in a pressure oil chamber formed between the plunger housing hole and a rear end of a plunger, to bias the plunger in a protruding direction, the chain tensioner allowing tension of a chain to be properly maintained.
2. Description of the Related Art
A chain tensioner is conventionally commonly used to properly maintain the tension of a chain. For example, a chain guide mechanism is well known in which a traveling guide shoe is used to slidably guide a transmission chain such as a roller chain endlessly passed between sprockets provided on a crank shaft and a cam shaft in an engine room and in which a chain tensioner is used to bias a rocking chain guide with the traveling guide shoe in order to properly maintain tension.
For example, a chain guide mechanism is configured such that a timing chain CH is endlessly passed between a driving sprocket S1 attached to a crank shaft in the engine room and a pair of driven sprockets S2 and S3 attached to a cam shaft in the engine room and such that the timing chain CH is guided by a rocking chain guide G1 and a fixed chain guide G2, as depicted in FIG. 10.
The fixed chain guide G2 is fixed in the engine room using two attachment shafts B1 and B2. The rocking chain guide G1 is attached to the inside of the engine room so as to be able to rock, being centered on an attachment shaft B0, in a passage plane for the timing chain CH.
A chain tensioner 500 presses the rocking chain guide G1 to properly maintain the tension of the timing chain CH while suppressing vibration.
The well-known tensioner 500 used for such a chain guide mechanism includes a tensioner body 510 having a cylindrical plunger-housing hole 511 that is open at one end, a cylindrical plunger 520 slidably inserted into a cylindrical surface portion 513 of the plunger housing hole 511, and bias means for biasing the plunger 520 in a protruding direction through the plunger housing hole 511, for example, as schematically depicted in FIG. 11.
The bias means includes a coil spring 540 that is housed in a tubular recess portion 521 of the cylindrical plunger 520 and that is compressed between the tubular recess portion 521 and a bottom portion 512 of the plunger housing hole 511.
Oil is fed through an oil feeding hole 514 formed in the plunger housing hole 511 to fill a pressure oil chamber 501 formed between the plunger housing hole 511 and the plunger 520. The oil causes the plunger 520 to be biased in the protruding direction, while a check valve 550 (the check valve 550 is schematically illustrated by depicting only a check ball) is used to inhibit outflow of the oil through the oil feeding hole 514.
Thus, as the plunger 520 makes reciprocating motion, the oil flows through a small gap between the plunger 520 and the plunger housing hole 511. The resultant channel resistance produces a damping effect that dampens the reciprocating motion of the plunger 520.
In the well-known chain tensioner, if an excessive tensile force is generated in the timing chain during use, the pressure in the pressure oil chamber becomes excessively high, possibly leading to noise, vibration, or damage to the timing chain.
For preventing the occurrence of such problem, a chain tensioner is well known which includes a relief valve that is opened when the pressure in the pressure oil chamber reaches a predetermined value or larger (see Japanese Patent Application Laid-open No. 2002-130401, 2002-235818, 2006-125430, or the like).
However, when the chain tensioners well known from Japanese Patent Application Laid-open Nos. 2002-130401, 2002-235818, 2006-125430, and the like, are left for a long time after stoppage of oil feeding, air is mixed into the pressure oil chamber, leading to an insufficient holding force at the time of startup.
Moreover, immediately after the startup, the feeding of the oil suffers a time lag, and thus, the oil in the pressure oil chamber only leaks and no new oil is fed, in spite of the reciprocating motion of the plunger. The oil in the pressure oil chamber decreases to prevent the damping force of the oil from acting on the plunger. Disadvantageously, the timing chain may vibrate significantly to produce an abnormal sound or may be damaged.